On my sailplanes I like Monokote opaque RED on the bottom of the entire wing and stab.

On the top I place Monokote opaque RED on the outer wing panels of the main wing and Monokote WHITE on the inner wing panels as well as the entire fuselage and the top of the stab.

What I find is that at a distance I can see the alternating soild red and then red/white quite easily when turning in circles and this is true for clear skies and overcast skies. When landing the white/red on the top contrast well with any background colors like green from trees and white from clouds on the horizon.

For high altitude work solid black on the bottom of the wing and stab is recommended by many people. I tend to agree. I can see a black hawk thermalling a great distance away.

As for 'orientation', you might mean 'seeing the plane' or you might mean 'detecting direction and speed'. The discussion above covers 'seeing the plane'. For determining 'direction and speed' I think that is harder at long distances and sometimes high altitudes.

When I think about those times when I became 'disoriented' I remember realizing that the main problem was that the plane was so high or far away that the there was very little angular distance change when making a turn. By that I mean even a full circle was performed in a fraction of a degree of vision. The circle was a normal size circle but at the distance it was observed it only accounted for a very small visual change. In those situations it was sometimes difficult to determine if the plane was coming toward me or going away from me at any part of the circle. After the addrenalin rush (panic) wore off I would have to fly 'straight' for a short while and then move the control stick to the left. If the plane actually looked like it turned in the correct direction then I felt I was again oriented. If it appeared to move in the opposite direction then I assumed the plane started the turn coming toward me so I would allow a half turn and then I felt oriented again. At these times the color was not a major issue as the plane was just a visual 'blob' with which I had to deal.

For flying under clouds I think the best color would be solid black under the wing and stab. I think that would give the best chance for keeping the plane visible. Someone generated a picture of a sailplane at various altitudes with two color schemes. One scheme was various (rainbow) colored wings and the other had black wings and stab. That was 'proof' to me that black was very, very helpful. Maybe someone has that image and can post it here. It is well worth looking at that picture. On your plane you could try black under the outer panels (tip area) and if you feel you need more then completely cover the bottom in black.

Followup: 'Tall Paul' included the image in his post below. It shows the black versus multi-color wings at various altitudes and sky colors. Very interesting.